Field
The present embodiment relates to vehicle wheels, and particularly to shields and devices used to reduce drag on rotating vehicle wheels.
Description of Prior Art
Inherently characteristic of rotating vehicle wheels, and particularly of spoked wheels, aerodynamic resistance, or parasitic drag, is an unwanted source of energy loss in propelling a vehicle. Parasitic drag on a wheel includes viscous drag components of form (or pressure) drag and frictional drag. Form drag on a wheel generally arises from the circular profile of a wheel moving though air at the velocity of the vehicle. The displacement of air around a moving object creates a difference in pressure between the forward and trailing surfaces, resulting in a drag force that is highly dependent on the relative wind speed acting thereon. Streamlining the wheel surfaces can reduce the pressure differential, reducing form drag.
Frictional drag forces also depend on the speed of wind impinging exposed surfaces, and arise from the contact of air moving over surfaces. Both of these types of drag forces arise generally in proportion to the square of the relative wind speed, per the drag equation. Streamlined design profiles are generally employed to reduce both of these components of drag force.
The unique geometry of a wheel used on a vehicle includes motion both in translation and in rotation; the entire circular outline of the wheel translates at the vehicle speed, and the wheel rotates about the axle at a rate consistent with the vehicle speed. Form drag forces arising from the moving outline are apparent, as the translational motion of the wheel rim must displace air immediately in front of the wheel (and replace air immediately behind it). These form drag forces arising across the entire vertical profile of the wheel are therefore generally related to the velocity of the vehicle.
As the forward profile of a wheel facing the direction of vehicle motion is generally symmetric in shape, and as the circular outline of a wheel rim moves forward at the speed of the vehicle, these form drag forces are often considered uniformly distributed across the entire forward facing profile of a moving wheel (although streamlined cycle rims can affect this distribution somewhat). This uniform distribution of pressure force is generally considered centered on the forward vertical wheel profile, and thereby in direct opposition to the propulsive force applied at the axle, as illustrated in FIG. 24.
However, as will be shown, frictional drag forces are not uniformly distributed with elevation on the wheel, as they are not uniformly related to the speed of the moving outline of the wheel rim. Instead, frictional drag forces on the wheel surfaces are highly variable and depend on their elevation above the ground. Frictional drag must be considered separate from form drag forces, and can be more significant sources of overall drag on the wheel and, as will be shown, thereby on the vehicle.
The motion of wheel spokes through air creates considerable drag, especially at higher relative wind speeds. This energy loss is particularly critical in both bicycle locomotion and in high-speed vehicle locomotion. Previous efforts to reduce this energy loss in bicycle wheels have included bladed-spoke designs; the addition of various coverings attached directly to the wheel; and the use of deeper, stiffer, and heavier aerodynamic rims. As winds, and particularly headwinds, are a principal source of energy loss in bicycle locomotion, expensive aerodynamic wheel designs have become increasingly popular. However, these aerodynamic wheel designs have often been tuned to reduce form drag, rather than frictional drag. As a result, augmented frictional drag forces present on these larger-surfaced aerodynamic wheel designs tend to offset much of the gains from reduced form drag forces, thereby negating potential reductions in overall drag.
Bladed spokes, tapered in the direction of motion through the wind, are designed to reduce form drag. These streamlined spokes suffer from increased design complexity, increased weight and higher costs. In addition, such bladed designs are more susceptible to crosswind drag effects: The increased surface area of the bladed spoke can rapidly increase form drag in the presence of any crosswind; any crosswind directed upon the flat portion of the spoke quickly increases pressure drag upon the spoke.
Under low crosswinds, the bladed spoke presents a relatively small forward profile facing oncoming headwinds, minimizing form drag. Indeed, the thin profile of the blade generally minimizes form drag over that of round spoke profile. However, most external winds will not be precisely aligned co-directional with the forward motion of the wheel. Such winds cause a crosswind component to be exerted upon the wheel, leading to flow-separation—and thus turbulence—behind the bladed spoke, and thereby generally negate the potential aerodynamic benefit of the bladed-spoke design. Under high crosswinds, the round spoke profile may even outperform the bladed spoke in terms of drag reduction. Perhaps a result of these conflicting factors, the bladed spoke has not become the common standard for use in all bicycle competitions.
Wheel covers generally include a smooth covering material attached directly to the wheel over the outside of the spokes, generally covering a large portion of the wheel assembly, often extending from the wheel rim to the axle. Wheel covers add weight to the wheel assembly and can result in more wheel surface area being exposed to winds. The additional weight on the wheel is detrimental to wheel acceleration, while the large surface area of the cover can increase frictional drag. Although covering the wheel spokes can reduce form drag forces thereon, the increased frictional drag forces on the larger surface areas can largely offset any aerodynamic benefit. In addition, covering large portions of the wheel also increases bicycle susceptibility to crosswind forces, destabilizing the rider. For this reason, wheel covers are generally used only on the rear wheel of a bicycle, and generally only under low crosswind conditions. Perhaps as a result of these conflicting factors, wheel covers have not become the standard equipment for use in all bicycle competitions.
Recently developed for use on bicycles, deeper, stiffer and heavier aerodynamic wheel rims suffer several drawbacks: deeper (wider along the radial direction of the wheel) and streamlined rims are often used to reduce profile drag on high-performance bicycle wheels. As mentioned, these rims are generally designed to reduce profile drag under various crosswind conditions. However, these deeper rims—having generally larger rotating surface areas—can dramatically increase friction drag. As will be shown, friction drag is particularly increased on the expanded upper wheel surfaces, largely negating any potential benefit of the reduced profile drag. In addition, such deep wheel rims with minimal spokes must be made stronger and stiffer—typically with double-wall construction—than conventional single-wall, thin-rim designs. As a result, such deep rims often ride more harshly over bumpy terrain, and are generally heavier, adding weight to the bicycle, which becomes a drawback when the grade becomes even slightly uphill.
As a result of these and other countervailing factors, no single wheel design has emerged as the preferred choice for reducing drag on bicycle wheels over a wide range of operating conditions. Instead, a variety of wheel designs are often employed in modern racing bicycles. In the same competition, for example, some riders may choose to use bladed spokes, while others choose round spokes, while still others choose deep rims or wheel covers. The differences in performance between these various wheel designs appear to only marginal affect the outcome of most races.
In sports cars, wheel covers have been employed to reduce aerodynamic vortices from developing inside the inner wheel assembly. These covers smooth the air flowing over the outer wheel and deflect a portion of the air to the brake linings, providing cooling thereon. In addition, various wings attached to the body of the car have been employed to deflect air around the drag-inducing exposed wheels. Such wings are generally located low to the ground, and are often configured to deflect air to one side or the other of exposed wheels, or to provide vehicle down-forces to counteract significant lifting forces generated by the exposed wheels. As will be shown, use of these wings to deflect air upwards onto the upper wheel can actually augment the down force problem, as well as contribute to more overall vehicle drag. And related flip-up deflectors have been incorporated into the body molding of some sports cars, although these have been generally limited to placement in front of the semi-exposed rearmost wheel. Such flip-up deflectors have also been used to augment down-forces in order to enhance vehicle stability at high speeds.
In various cycles, fenders and mud-covers have been used to cover wheels for other purposes. However, these items are generally oriented on the cycle consistent with their intended purpose of shielding the rider from debris ejected from the wheel. As such, they are not necessarily designed to be either forwardly positioned, nor closely fitted to the tire and wheel for aerodynamic shielding purposes. On some bicycles, skirt guards have been employed specifically to prevent clothing of the rider from becoming entangled with the rotating wheel. However, these guards are often made of porous construction, and are generally employed on the rear-most wheel, rather than on the front-most wheel, where the potential aerodynamic benefit is generally greater.
Perhaps because aerodynamic devices are generally not allowed by rules governing many bicycle competitions, development of fairings for bicycles remains somewhat limited. Instead, fairings have been generally used to cover either the entire cycle, or the broad front area of the cycle, shielding both rider and cycle. Enclosing-type fairings typically have quite large surface areas, which augment frictional drag forces, largely negating any benefit in reducing form drag from streamlining the forward profile of the bicycle. Nevertheless, numerous bicycle speed records have been achieved using these larger fairings, validating their effectiveness. Frontal wind-deflecting fairings are typically used to reduce form drag on various components on a cycle; however, their greatly expanded surface areas can minimize their effectiveness by introducing greater frictional drag. The potential effectiveness of using smaller fairings—having minimal form and friction drag—for shielding specific, critical, drag-sensitive areas of moving wheel surfaces has not been properly recognized.
A study by Sunter and Sayers (2001), Aerodynamic Drag Mountain Bike Tyres, Sports Engineering, 4, 63-73, proposed and tested the use of a front-mounted wind-deflector fender for relatively low-speed, rough-surfaced, down-hill racing mountain bicycle front wheels. However, as will be shown, the tested fender was unnecessarily extensive; its extended design—covering the tire to well below the level of the axle—failed to focus properly on key sources of drag on a typical bicycle wheel. Instead, in this investigation, variations in drag were measured with differing tire tread patterns, and differing fender clearances, using knobby mountain bike tires, and were measured on the front wheel only. Moreover, sufficient fender clearances with the tire were investigated, with the aim of determining any potential benefit in reducing drag on the bicycle against the potential mud accumulation there-between.
Referencing an earlier study, Kyle (1985) Aerodynamic Wheels. Bicycling, December, 121-124, in this later study, Sunter and Sayers noted a 30% increase in drag on a wheel rotating with a speed equivalent to the exposed headwind, versus a stationary wheel exposed to the same headwind. As reported, this measurement seems to have represented the increase in torque needed to rotate the wheel about the axle. However, the change in torque measured about the axle on a fixed wheel mounted in an air-stream—as will be shown—cannot be considered an accurate representation of the change in drag force required to propel the bicycle. Torque measured this way is only an indirect factor needed to determine the effects on overall bicycle drag. As will be shown, the net drag force is generally not well centered on the rotating bicycle wheel, causing drag forces on the upper wheel to be magnified. Indeed, the offset drag force on the wheel contributes significantly more to overall bicycle drag than commonly understood.
A number of studies of bicycle wheel drag measured in wind tunnels also fail recognize the importance of drag forces on the upper wheel. Tests are typically conducted with the wheel suspended in the airstream, with the drag on the wheel measured via force gauges attached to the suspension arm. As will be shown, the magnification of upper wheel drag forces occurs when the wheel is in contact with the ground. Measuring drag on wheels suspended in an airstream will yield incomplete results, particularly for application to moving vehicles.
For example, an earlier study by Greenwell et al, Aerodynamic characteristics of low-drag bicycle wheels, Aeronautical Journal, 1995, 99, 109-120, measured translational drag on a wheel suspended from a torsion tube in a wind tunnel, where the wheel was driven by a motor and made no contact with a ground plane. They concluded that in this configuration—unexpectedly—rotational speed had little influence on the translational drag force directed upon the wheel assembly.
In a more recent study, Moore and Bloomfield, Translational and rotational aerodynamic drag of composite construction bicycle wheels, Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology Jun. 1, 2008, vol. 222, no. 2, 91-102, the measured drag was extended to include rotational drag on the wheel. However, this study also failed to include a ground plane in contact with the wheel; the wheel remained suspended wind tunnel. As mentioned, this configuration does not accurately reflect the total retarding force upon a vehicle in motion caused by drag forces on the wheel.
Sunter and Sayers also failed to recognize the magnifying effect that an off-center net drag force on the wheel can have on overall bicycle drag. Instead, they concluded that with the modest improvement in drag torque measured upon the rotating wheel using the wind-deflecting fender, only corresponding modest improvement in overall bicycle drag could be expected. They further concluded that the use of extensive front-wheel wind-deflecting fenders—having a rather large forward profiles—might thus prove beneficial in the specific application of mountain bicycle downhill racing, where only modest reductions in overall drag might yield a winning advantage in higher speed races. This conclusion would be consistent with the faulty observation that total drag forces are generally well centered on the wheel.
It has long been recognized that minimizing drag on large trucks and their trailers has significant potential for improving fuel economies. Extensive wind deflectors have been used in front of the rear wheels on extended truck trailers, but are generally designed to deflect very large volumes of air to the either side of the rear wheels. Deflecting unnecessary volumes of air with large fairings, can produce significant form drag by the fairing itself, thereby negating much of the intended benefit in reducing wheel drag. And as will be shown, deflecting air below the level of the axle can be particularly detrimental in reducing overall vehicle drag.
For example, in patent US 2010/0327625A1, a V-shaped air deflector is shown positioned in front of a wheel for the purpose of deflecting large volumes of air to either side in order to reduce wheel drag. However, the deflector shown is unnecessarily large, introducing substantial form drag. And the deflector is positioned centered at the elevation of the axle, extending symmetrically above and below the axle. As mentioned, deflecting air onto lower surfaces of the wheel can be detrimental to reducing overall drag on the vehicle. And as shown, the deflector fails to fully shield the most critical drag-inducing uppermost surfaces of the wheel.
In U.S. Pat. No. 6,974,178B2, the deflectors shown are simply unnecessarily large, again introducing substantial form drag.
Other examples in the art also deflect air downward onto the lower surfaces of the wheel, thereby negating much of the intended aerodynamic benefit. These include trailer wheel deflectors of patents US 2010/0066123A1, U.S. Pat. Nos. 4,640,541, 4,486,046 and 4,262,953.
Other examples in the art also shield headwinds from lower surfaces of the wheel below the axle, thereby also negating much of the intended aerodynamic benefit. These include fairings of patents US 2011/0080019A1, U.S. Pat. No. 7,520,534B2, U.S. Pat. No. 7,322,592B2, U.S. Des. 377,158, U.S. Pat. Nos. 5,348,328, 4,773,663, 4,411,443, 4,326,728 and 2,460,349.
Wheel fairings often used on light aircraft are generally designed for reduced form drag of the wheel assembly while airborne, and generally cover the upper wheel to well below the axle. An example is shown in U.S. Pat. No. 4,027,836. Wheel pants designed as mud covers for shielding wings from ejected debris are also seen in the art, showing an upper wheel fender designed with extensive streamlined surfaces often extending substantially above and behind the wheel. As will be shown, such designs are not optimized for terrestrial use, either by extending below the level of the axle, or by not optimally shielding the most critical drag-inducing surfaces of the upper wheel.
Other examples in the art show skirt guards on the rear wheel of cycles, where the potential aerodynamic benefit is considerably less than that of the front wheel. These include guards of patents U.S. D634,249S, U.S. Pat. Nos. 3,101,163 and 1,027,806. And fender of patent U.S. D612,781S is not shown mounted on a vehicle.
Spoke art includes many examples having rectangular or otherwise non-aerodynamic cross-sectional profiles of wheel spokes for use in automotive applications. Examples include patents U.S. D460,942, U.S. D451,877, U.S. D673,494, U.S. D396,441 and others.
Cycle spoke art includes a tapered spoke of U.S. Pat. No. 5,779,323 where the cross-sectional profile of the spoke changes from more highly elliptical near the wheel hub to more generally oval near the wheel rim. As will be shown, the spoke shown is tapered to minimize—rather than maximize—any aerodynamic benefit, especially when used in the presence of crosswinds. Tires are generally designed with tread patterns intended to maximize traction with the road surface and to minimize rolling friction and road noise. A wide variety of tread patterns exist, each designed for specific vehicle applications. Some tires with aggressive tread patterns are designed for maximum traction in off-road conditions. These tires generally have square or rectangular lugged patterns in the tread, and suffer from increased road noise and wind resistance. The need for tires with aggressive tread patterns specifically designed to reduce drag on the vehicle have been largely overlooked.
For example, in US patent 2007/0151644 A1, an oval-shaped tread pattern is shown. While oval shaped tread lugs have the potential to reduce drag, their closely spaced pattern shown diverts most of the air to flow over the top surfaces of the tire, rather than between the lugs. Moreover, the closely spaced oval lugs are designed to reduce stresses during tire compression—thereby improving the rolling resistance of the tire—and to reduce the incidence of trapping stones between the lugs. As such, the minimally spaced lugs are an essential characteristic of this embodiment. As disclosed, any minor improvement in the aerodynamic characteristics of the tire was not included.